The summer diet of fish-eating killer whales in the Avacha Gulf of Kamchatka: Are there any preferences?

Dietary specialization has led to the origin of several ecotypes of killer whales (Orcinus orca). Comparing the feeding habits of different populations within an ecotype is valuable for understanding the evolution of foraging strategies. In this study we investigated the diet of fish-eating killer whales in the Northwest Pacific Ocean (Avacha Gulf, Kamchatka) and compared it to the published data on Northeast Pacific fish-eating killer whales. We collected prey fragments (N = 141) after hunting events in the summer seasons of 2011–2016 and determined the species of prey. Killer whales fed mainly on various species of Pacific salmon in Avacha Gulf. Coho salmon (Oncorhynchus kisutch) (56% of samples) and chum salmon (O. keta) (31% of samples) predominated in the diet. The results are comparable to the peculiarities of the fish-eating killer whale diet in the Northeast Pacific where killer whales prefer Chinook salmon (O. tshawytscha) most of the summer but shift toward coho and chum in late summer. Avacha Gulf could be an area where killer whales come to hunt coho similar to Prince William Sound in Alaska where killer whales prefer coho to other salmon species. We found significant differences in the diet between sex-age categories. Females (N = 42) and juveniles (N = 15) were more selective and hunted mainly coho while males (N = 53) hunted chum equally to coho. Like Chinook in British Columbia, coho in Kamchatka has a prolonged spawning period. Coho is the most nutritious after Chinook. Coho’s prolonged abundance and nutritional value probably make it preferable to killer whales especially females.